fix(dashboard): treat chart ID 0 as valid in useDashboardFormData

The early-return null check used `!chartId`, which incorrectly
short-circuits for a falsy but valid chart ID of 0. Use an explicit
null/undefined check instead.

Co-Authored-By: Claude <noreply@anthropic.com>

superset-frontend/src/dashboard/hooks/useDashboardFormData.ts

@@ -61,7 +61,7 @@ export const useDashboardFormData = (
 
     // Early return if we don't have required data or chartId
     if (
-      !chartId ||
+      chartId == null ||
       !nativeFilters ||
       !dataMask ||
       !chartConfiguration ||